(a) Field of the Invention
This invention relates to a lens working apparatus for profiling a lens blank to form a circumferential rib on the edge of the lens blank.
(b) Description of the Prior Art
In general, spectacle lens are formed by profiling a model to work the new lens into exactly the same shape as that of a corresponding rim of a spectacle frame. The thus obtained lens is fitted in a rim by interengagement of a V-shaped groove on the inner edge of the rim and the formed circumferential rib on the edge of the lens.
The groove on the inner edge of the spectacle frame rim is formed to have a spherical surface of a radius of curvature of the defined 5.5 curve (95.1 mm) as standard, but it is difficult to form a circumferential rib of such radius of curvature on the lens edge. Therefore, prior art working has been carried out with a radius of curvature only set approximately. In particular, prior art working into a noncircular but irregular shape with variation in the distance from the axis of the lens to the contact point of the grindstone with the edge of the lens was performed with difficulty resulting in a circumferential rib having unsmooth or, in extreme caes, stepped curvature at specified positions.
The prior art lens worked under the above-mentioned conditions was fitted in the spectacle frame rim inevitably with required extra steps of, for example, folding correctively the rim or manually correcting the rim shape.